The present invention is based on a device for determining at least one parameter of a medium flowing in a line, including a sensor carrier for accommodating a sensor element.
German Published Patent Application No. 44 26 102 and U.S. Pat. No. 5,693,879 describe a sensor carrier for a sensor element in an air-mass measuring device, the sensor carrier extending with the sensor element into a measuring channel in which a medium is flowing. The sensor element supplies a measuring signal used for calculating the mass of the flowing medium. The sensor carrier has a recess in which the sensor element is flush-mounted and retained by an adhesive layer applied on a bottom surface of the recess. In this case, the sensor carrier is produced in that first of all, an opening which corresponds approximately to the outer shape of the sensor element is made in a metal strip. The metal strip is thereupon bent about a bending axis outside of the recess and then pressed together so that a bent part of the metal strip forms a retaining element, and an unbent part of the metal strip having the opening forms a frame element of the sensor carrier. The retaining element covers the opening of the frame element and, together with the frame element, forms a recess. After that, further deforming of the retaining element produces plateau-like elevations which are used as spacers or bearing surfaces. The sensor element is then glued in place in the recess.
It is extremely important that the sensor element be glued in position in the recess with its top surface as flush as possible with respect to the top surface of the sensor carrier, since just the smallest displacement, e.g., because of an unevenly applied adhesive layer, results in eddies and flow separation regions which, particularly at the surface of the sensor element, disadvantageously influence the heat dissipation of the measuring resistor and invalidate the measuring result. Therefore, very small mass tolerances must be provided for the recess, and the most extreme care is necessary when gluing the sensor element into the recess of the sensor carrier, so that particularly when the device is mass-produced, a high degree of sophistication is necessary from a standpoint of production engineering, resulting in considerable production costs.
The various work steps for producing the frame and retaining elements are disadvantageous. In addition, the flowing medium may flow through the folding gap between the frame and retaining elements. However, this is not disadvantageous, since this effect may be suppressed by zero-point measuring and calibration. Although, during the service life of the sensor element, the measuring result is invalidated if this folding gap is obstructed by dirt and/or liquid particles, and the calibration is no longer correct.
It is disadvantageous that the spacers are first formed by a further shaping process. The tolerance of the depth dimension of the recess is given by the tolerance of the thickness of the metal strip and the tolerance of the folding gap thickness.
It is also disadvantageous that, because of the flowing corrosive medium, a corrosion-protection layer such as NiNiP must be applied on the sensor carrier by an additional costly electroplating process or a coating method which further increases the dimensional tolerances and the production times and costs.
When working with such a self-supporting manner of mounting the sensor element, because of tolerances during production, a gap develops between the sensor element and the recess of the sensor carrier. The gap may be so large that in the case of the sensor element, an unwanted flow in the hollow space below its membrane in the recess may come about, which has a disadvantageous effect on the measuring result of the device.
That is why devices are described with which the disturbing influence of the seepage may be reduced. A diversion of the flow at a specially formed edge of the sensor element, as described in German Published Patent Application No. 195 24 634 and U.S. Pat. No. 5,723,784, respectively, prevents the medium flowing in via the gap from getting into a hollow space below the membrane of the sensor element. An application of gluing seams, as described in German Published Patent Application No. 197 43 409, may prevent the penetration of the medium into the gap about the sensor element, in order to avoid unwanted seepage. Disadvantageous in both methods is that only by the special arrangement of gluing seams or by additional measures is the flow diverted around the hollow space in order to compensate for the effects of the manufacturing tolerances.
German Published Patent Application No. 197 44 997 describes a device which makes it possible to protect the components of an evaluation circuit as well as the connecting lines to the contacting region of the sensor element from moisture using a gel, and soiling of the sensor region, thus, the part of the sensor element where a membrane is located, by the gel is prevented. In that case, expansions of a gap, which extends between the sensor element and the walls of the recess, are provided, in order, with the aid of the expansions, to reliably stop a further flow of a protective layer, applied at least partially on the evaluation circuit, in the gap, so that the flow path of the protective layer always remains clearly defined. In this connection, the disadvantages from the standpoint of production engineering result, that gaps must additionally be created, the flow of the gel not being stopped, but rather only being diverted in a defined manner.
German Published Patent Application No. 198 28 629 describes a thermal air-flow sensor in which a carrier housing and a measuring housing are formed separately from one another, and the measuring housing and the carrier housing are cemented on a base plate element.
In contrast, the device according to the present invention may provide the advantage that in a simple manner, the measuring result may not be impaired even during longer operating time, because the measuring result is not influenced due to a flow under the measuring element by an air stream via an open or clogging folding gap, and according to the present invention, the tolerance of the depth dimension of the recess may be determined only by the tolerance of the sensor cavity, and no longer additionally by the tolerance of the folding gap.
It may be advantageous to secure the sensor carrier in a bypass channel or in a support part, since this simplifies the assembly.
If the sensor carrier is secured to a base member, a sensor element may be connected to electronic equipment prior to insertion into the device. An aerodynamically formed oncoming-flow edge may be advantageous for the oncoming-flow behavior.
For optimized circumflow of the sensor element, the sensor element may be installed flush with respect to a top surface of the sensor carrier, and/or if a small gap is present between the sensor element and the sensor cavity.
Plastic from the plastics class of liquid crystal polymers, or partial crystalline, aromatic thermoplastic may be used.
During assembly, an adhesive bead, which completely seals the sensor region of the sensor element in the sensor cavity, may be placed into the cut-outs in the longitudinal edges of the sensor cavity, transversely over the bottom of the sensor cavity, and depressions may be applied in the edge area of the sensor cavity bottom, so that the sensor element may be mounted more exactly. This adhesive bead prevents soiling of the sensor element by reliably stopping the gel which protects an evaluation circuit from moisture.
It may be advantageous to use plastic for the sensor carrier, since forms of a more filigree nature, and aerodynamic requirements such as those of the oncoming-flow edge may be taken into account by the possibilities for shaping the plastic in any manner desired.
Moreover, it may be advantageous to use plastic or ceramic, since plastic does not corrode so seriously compared to metal, and therefore no further corrosion protection may be necessary.
Since it may be possible to place the sensor element very precisely in the sensor cavity due to the narrowing of the tolerances because of the use of plastic, there may no be longer any flow below the sensor element.
Several example embodiments of the present invention are illustrated schematically in the drawings, and are explained more precisely in the description below.